1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a reverse link soft hand off method for improving a reverse link communication quality in a CDMA cellular communication system.
2. Background of the Related Art
In general, a cellular communication system is provided with a hand off function for permitting a mobile station to have a radio link kept continuously when the mobile station roams from an area of a serving base station giving a service presently to a service area of new base station. In the hand off which is a feature coming from a mobility of the mobile station, the mobile station shifts a call underway presently to a target base station by keep searching signal strengths of the serving base station giving a service to the mobile station presently and base stations around the mobile station and comparing the signal strengths of the searched base stations. Eventually, a good communication quality can be maintained because the mobile station is permitted to keep the communication through a communication channel secured newly according to the hand off function. In the hand off of the CDMA cellular communication system, there are a hard hand off a momentary cut off of the communication is occurred, and a soft hand-off call is continuous without cut off of the communication. The hard hand off is made mostly between switching centers between which no radio link for the soft hand off is not established, and the soft hand off is made between a plurality of base stations around a mobile station having identical frequencies assigned thereto. A radio link from the base station to the mobile station is defined to be a forward link and a radio link from the mobile station to the base station is defined to be a reverse link.
FIG. 1 illustrates a part of a CDMA cellular communication system, referring to which a related art soft hand off will be explained.
CDMA channels having identical frequencies assigned thereto, but having offsets different from each other are used between a mobile station 1 and each base station 2, 3 and 4. The mobile station 1 keeps tracing a pilot channel in the CDMA channels of the forward link, and the each base station 2, 3 and 4 transmits a pilot signal to the mobile station 1 through pilot channels of sequence offsets different from each other. Then, the mobile station 1 respectively measures pilot signal strengths of forward links received from the base stations 3 and 4 around the mobile station 1 inclusive of the first base station 2 which is a serving base station to the mobile station presently, and compares to a preset threshold. The preset threshold is an ADD threshold and a DROP threshold. The ADD threshold is a lowest base station signal strength at which an available communication channel can be sustained, and the DROP threshold is a highest base station signal strength at which release of an unavailable communication channel is required. The mobile station 1 transmits a signal strength message to the first base station 2 when the mobile station 1 finds out that the pilot signal strength of the first base station 2 drops below the DROP threshold as a result of comparison of the pilot signal strengths of the forward link to the preset thresholds or finds out a pilot signal having a strength substantially higher than a strength of the pilot signal from the first base station 2. If the pilot signal strength of the first base station 2 drops below the DROP threshold and only the pilot signal strength of the second base station among the pilot signal strengths of the forward links measured at the mobile station 13 is higher than the ADD threshold on the same time, the mobile station 1 transmits a signal strength message having the pilot signal strength of the second base station 3 recorded thereon to the first base station 2. Then, the first base station 2 reports the received signal strength message to a base station controller 10, and a call control processor 11 in the base station controller 10 analyzes the received signal strength message. The call control processor 11 instructs a soft hand off for the mobile station to the first base station 2 and the second base station 3 according to a result of the signal strength message analysis. At the end, the communication channel between the mobile station 1 and the first base station 2 is released and a new communication channel between the mobile station 1 and the second base station 3 is established. In this instance, the second base station 3 is a target base station for the soft hand off of the mobile station 1.
The procedure of the soft hand off explained up to now is on a forward link soft hand off, in which both direction hand off is conducted upon reception of the hand off message based on the forward link pilot signal strength. Thus, the CDMA cellular communication system has further improved a forward link communication quality by the forward link soft hand off. Opposite to this, it is generally accepted fact that a reverse link has a communication quality lower than the forward link due to a shadow effect, a corner effect, and a scattering effect. In other words, the reverse link has a communication quality significantly lower than in the case of the forward link because the communication quality is affected by the shadow effect occurred when the mobile station is surrounded by obstacles such as high buildings, the corner effect occurred when the mobile station turns around a corner, or the scattering effect in which a signal is scattered irregularly in the vicinity of the mobile station. For example, if a particular mobile station transmits a signal to a serving base station during moving in a region surrounded by high buildings, though the signal is attenuated by the shadow effect, other signals from other mobile stations around the particular mobile station, which act as noises to the signal from the particular mobile station, are not attenuated, resulting in an inevitable deterioration of the reverse link communication quality as a signal to noise ratio of the reverse link is dropped in view of the serving base station. Nevertheless, as explained, even though the related art CDMA only has employed the forward link soft hand off in which the soft hand off is made for both directions based on the forward link pilot signal strength, no soft hand off based on a reverse link mobile station signal strength has been suggested.